Just commit, no matter the response. Adding /s is akin to explaining a joke after you tell it. If someone doesn’t understand or pick up on the sarcasm, it’s not your responsibility to spell it out for them. You add /s because you don’t want to be held accountable.
What qualifies as “useless shit” is to be determined, but I agree with you. If you need to use /s, your quip isn’t as cunning or pointed as you think.